Various catalysts are known to promote the "ene reaction" of olefins with alpha-, beta-unsaturated carboxylic acid esters for the production of unsaturated carboxylic acid esters. For instance, U.S. Pat. No. 3,783,136 and German Offenlengungsschrift both describe the use of AlCl.sub.3 and AlBr.sub.3 as catalysts for such reactions. U.S. Pat. No. 4,506,095 describes the reaction of linear alpha-olefins with alkyl acrylates catalyzed by an organometallic catalyst of the formula Rn-Al-X.sub.3-n, wherein R is an organic radical containing between about 1 and 12 carbon atoms, n is the integer 1 or 2, and X is chlorine or bromine. U.S. Pat. No. 3,641,120 describes the reaction of an ester with an olefin in the presence of a combination of a manganic carboxylic acid salt or oxide with a zirconyl carboxylic acid salt or zirconium oxide. The publication by B. R. Snider on J. Org. Chem., Vol. 39, No. 2 (1972), p. 255, refers generally to Lewis acid catalysts for ene reactions, but illustrates only the use of aluminum chloride and zinc bromide. U.S. Pat. No. 3,855,255 describes the reaction of carboxylic acid esters by reacting diolefins with methacrylate esters in the presence of an organometallic complex of zero-valent nickel and an electron donor. U.S. Pat. No. 2,093,695 discloses preparation of carboxylic acid esters by reaction of acyloxy compounds with olefinic hydrocarbons catalyzed by activated charcoal, inorganic acids, the halogens and various halides of calcium, boron, cadmium, zinc, calcium and potassium. Co-pending Application Ser. No. 510,309, filed Apr. 17, 1990, U.S. Pat. No. 5066829 describes an ene reaction catalyzed by tantalum pentachloride. Akermark et al (J. Org. Chem., Vol. 43, No. 22 (1978), 4387) have reported that the eutectic mixture of AlCl.sub.3 NaCl, and KCl is a superior ene reaction catalyst. U.S. Pat. No. 3,892,788 teaches a ligand-stabilized Pt(II) dihalide complex combined with a Group IVB metal halide as a catalyst for such ene reactions. South African Patent 496,265 describes one such reaction catalyzed by various organo-metal compounds. According to German Offenlegungsschrift 2044159, a reaction between acrylic acid esters and dienes is catalyzed by an organometallic complex of zero-valent iron and a triaryl compound of an element of Group V. U.S. Pat. Nos. 3,755,386 and 4,144,257 and Belgian published application 739,208 describe similar reactions using complexes of Group VIII compounds, as well as various compounds and complexes of iron, nickel and cobalt. U.S. Pat. Nos. 4,009,203 and 3,534,087, German Offenlegungsschrift 3149979 and World Patent No. 8100846 describe related reactions of acids and olefins catalyzed by and acyloxy-stannic trihalide or a perfluorosulfonic acid resin or a crystalline metal silicate or an aluminum silicate containing a Group VIII metal compound and a polyvalent metal halide.
It has been found that a co-catalyst comprising a mixture of aluminum chloride and ethylaluminum dichloride exhibits the high activity which would be obtained by using aluminum chloride alone as well as serving to suppress the formation of alkyl chloride by-products which is characteristic of conventional aluminum chloride-catalyzed processes.